1. Field of the Invention
The present invention relates to a method for cutting a more or less rectangular piece of sheet metal that is welded onto a wall of an atomic fuel assembly storage rack cell such that a layer of neutron-absorbing material, held initially between the sheet metal and the wall, may be removed.
This type of method is used to refurbish the atomic fuel assembly storage racks by allowing new plates of neutron-absorbing material to be subsequently installed against the walls.
2. Background Art
In the nuclear industry atomic fuel assemblies are generally stored in storage racks placed in pools in which coolant water circulates.
This type of installation houses either new atomic fuel assemblies or atomic fuel assemblies that have been previously irradiated in the core of the nuclear reactor. This type of installation may be used on the actual site of the reactor or on other sites such as in reprocessing plants.
The storage racks comprise a rigid structure that contains a certain number of juxtaposed compartments the longitudinal axis of which is vertical. Each compartment, also called xe2x80x9ccellxe2x80x9d, is intended to house an atomic fuel assembly. To this end the cross-section of the cells is similar to that of the assemblies, i.e. generally square.
The rigid structure of the storage racks can be achieved in a variety of ways. The partitions that define the cells can be obtained, for example, either by using welded feet to assemble tubes with a square cross-section or by welding parallel plates that are at right angles to one another.
Whatever technique is used to produce the rigid structure of the rack all or some of the partitions that separate the cells are generally fitted with a neutron-absorbing material that is available in a variety of forms. This neutron-absorbing material is used to absorb neutrons emitted by the fissile material contained in atomic fuel assemblies, thus reducing the distance that separates two adjoining cells to a minimum and thereby optimizing the number of atomic fuel assemblies that can be stored in a pool of given measurements.
One of the techniques used to integrate neutron-absorbing material in the partitions that separate the cells consists in placing a layer of neutron-absorbing material between each wall of the rigid structure and welding a piece of sheet metal onto said wall.
In this configuration the layer of neutron-absorbing material can be constituted by a polymer loaded with natural boron. Neutron-absorbing material of this type is marketed under the name xe2x80x9cBoraflexxe2x80x9d.
Old storage racks fitted with such neutron-absorbing material show significant signs of aging. This aging is due to the neutron-absorbing material dissolving in the pool water when it is subjected to a high degree of radiation caused by the atomic fuel assemblies.
One of the main consequences of this deterioration in the neutron-absorbing material is a reduction in the nuclear protection that it ensured initially. It is no longer, therefore, possible to maintain the initial storage density after the material has been used for a certain length of time.
The second drawback of the neutron-absorbing material being dissolved in the pool is that it pollutes the cooling system water to an excessive degree. After the material has been used for a certain length of time the pollution reaches levels that are not acceptable under normal conditions.
The standard solution consists in replacing the rack assembly when this situation occurs in storage racks fitted with this type of neutron-absorbing material.
The inventors of European patent A-0 896 339 suggest refurbishing old storage racks built in this way by cutting the sheet metal that holds the layers of neutron-absorbing material. The sheet metal is cut along the welds, the layers of neutron-absorbing material are removed and new plates of neutron-absorbing material are set in place against the walls of the rack cells. More precisely the sheet metal is cut using a water jet or by being chipped.
This solution is an ideal alternative to replacing the rack assembly as it enables the assembly to be refurbished by replacing the layers of neutron-absorbing material while retaining its rigid structure.
However, the techniques recommended in the patent to cut the sheet metal present major drawbacks.
The use of a water jet to cut the sheet metal requires a certain degree of accuracy in the relative position of the cutting head and the sheet metal which is difficult to ensure in practice due to the uncontrollable changes that occur in the shape of the sheet metal over time. Furthermore, this technique creates liquid and solid waste that has to be treated.
The cutting technique using chipping does not present the above-mentioned drawbacks. Nevertheless, it is sometimes impossible to use this technique to cut the long sides of the sheet metal that hold the layers of neutron-absorbing material due to the space required by the tooling when operating. This is particularly the case in racks where pieces of sheet metal are cut that face each other and are only separated by limited spaces.
The aim of the invention is a method for cutting a piece of sheet metal used to hold a layer of neutron-absorbing material, said piece of sheet metal being welded onto a wall of an atomic fuel assembly storage rack cell. The method is used to refurbish a rack of this type and does not present the drawbacks involved when the water-jet or chipping techniques suggested in European patent A-0 896 339 are used.
According to the invention these results are obtained using a method for cutting a more or less rectangular piece of sheet metal the two short sides and two long sides of which are welded onto a surface of an atomic fuel assembly storage rack cell. This method removes a layer of neutron-absorbing material initially held between the sheet metal and said surface. The method is characterized in that it comprises the following stages:
cutting the two short sides of the sheet metal;
cutting the two long sides of the sheet metal in a continuous movement from one end of the sheet metal to the other with a cutting head that includes at least one blade-blade support assembly or at least one single blade.
Preferably, both long sides of the sheet metal are cut simultaneously with a cutting head that includes two blade-blade support assemblies or two single blades.
In one version the two long sides of the sheet metal can also be cut by making two successive passes with a cutting head that includes at least one single blade-blade support assembly or one single blade.
In another version the two long sides of two pieces of sheet metal facing each other can be cut simultaneously in a single pass with a cutting head that includes four blade-blade support assemblies or four single blades.
According to requirements each blade is either fixed or movable and operates in an alternating or rotating movement when the cutting head is displaced.
Furthermore, each blade-blade support assembly of the cutting head comprises either a fixed blade holder, a roller or similar movable blade support that is caused to rotate when the cutting head is displaced, or a movable blade support that operates with an alternating movement.
In all these configurations after the two long sides of the sheet metal have been cut the two remaining side strips are advantageously lifted by at least one lifting device, such as a base or roller, that is borne by the cutting head.
The sheet metal to be cut is generally welded onto the wall with welding spots. In this case the short sides of this sheet metal may be cut along the welding spots on said short sides either by chipping, grinding or milling.
In one version the two short sides of the sheet. metal can also be cut along the entire width of the sheet metal by grinding- or milling-off sections.